This invention relates to a method and apparatus for correcting an error which is a function of an independent variable such as errors in analytic instruments in general and more particularly to an improved system for maintaining below range synchronization in such apparatus.
Various types of analytic instruments have a base line which varies with a change in an independent variable. Furthermore, certain instruments have errors which change not only as a function of the independent variable but which also change over periods of time, with temperature, etc. This is a particular problem in instruments such as dual beam spectrophotometers. It is also a problem in scanning calorimeters. In the first case, an output signal error must be corrected synchronously with changes in wavelength. In the second case, corrections must be made to the output as the independent variable of temperature changes. In order to gain a better understanding of the present invention it will be disclosed in terms of a dual beam spectrophotometer. It will be understood by those skilled in the art that it may be as easily used with any type of instrument or device in which an error which is a function of an independent variable occurs.
In a dual beam spectrophotometer the concentrations of various constituents in sample substances are determined. To accomplish this two radiation beams from a single source are sequentially directed to a photo-electric detector. One of the beams, whose signal is designated I, passes through the sample. The other signal, designated I.sub.0 does not pass through the sample but provides a reference. The I beam which is passed through the sample experiences a decrease in intensity due to absorption by the constituent in proportion to its concentration in the sample. Ideally, except for the absorption by the sample, the I and I.sub.0 beam have equal intensity throughout their transmission paths. This is commonly expressed as a ration I/I.sub.0 = 100%. However, the paths never include exactly the same optical elements since it is impossible to exactly match the reflectivity of the uncommon elements for all wavelengths of the source. In a spectrophotometer wavelengths are scanned in increments, the instrument stepping through the various wavelengths. There are variations in the transmission at each of these wavelengths. Without a correction, the base line, i.e, the zero line from which intensity is measured, is not flat and erronous results are obtained. This is a problem which has been recognized in the art and is referred to as a base line flattening problem. Various attempts have been made to solve this problem through the use of cams, tapped potentiometers or even through the use of the magnetic tape as a medium for storing the I.sub.0 correction factor which varies in synchronism with changes in wavelength. The cam and tapped potentiometer methods are tedious to adjust and force the user to accept predetermined inflection points not too closely spaced with respect to wavelength function. The magnetic tape method is capable of automatically finding or adjusting the correction function and has no restriction on the occurrence of inflection points. However, it is an expensive method in view of the requirements for synchronizing the tape drive to the wavelength drive in the instrument.
Co-pending application Ser. No. 654,704 describes and claims an improved error correcting system for use in such applications.
It discloses a device in which an independent variable is stepped or scanned between a first limit and second limit and in which device there is an error which is a function of the independent variable. It provides a manner of correcting that error by storing correction values for each of a plurality discrete steps of the independent variable, establishing a location marker which can be identified as different from the error correction data and by moving the location marker so that it is always adjacent the error correction for the current position or current step of the independent variable. The data adjacent the location marker is then read out to provide the correction.
Preferably, the data is stored in digital form in a shift register or similar device with the bit pattern of the location marker different than any data bit pattern corresponding to an error correction value.
In the preferred embodiment of the co-pending application in addition to the location marker, an index marker having a pattern different from the location marker and also different from any data pattern is also provided. The location marker, at one end or one limit is placed one data position away from the index marker and is moved further away from the index marker as the independent variable is stepped toward the second limit and toward the index marker when the independent variable is stepped in a direction from the second limit toward the first limit.
In order to attain the maximum amount of correction with the minimum of amount of hardware, preferably only incremental changes in error are stored. This permits two bits to comprise each storage position or location with a "1 0" indicating a change in the positive direction, a "0 1" an incremental change in the opposite direction and a "0 0" indicating no change. This also permits establishing a location marker as the bit pattern "1 1" and the index marker as a bit pattern "1 1, 1 1 " taking up two storage positions. A storage position is defined as the number of bits necessary to store the correction for one step of the independent variable.
In the disclosed system, calibration may be done between first and second limits which do not correspond to the full range of the device. Thus, after calibration, operation below the calibrated range is possible. Thus, there is a need to insure that synchronization is maintained when going below the calibration range.